measures around 430ppm, which is the same as
outdoor air.
Figure
B
shows readings taken over a 3-hour
period in a classroom. Cross-ventilation means
that a door and window are open and the wind is
blowing through; Windows only means that only
the windows are tilted open a bit.
In Figure B, the CO
2
level rises smoothly to
about 1000ppm, a level thats widely recognized
as a threshold for taking action to improve
ventilation. This doesn’t mean CO
2
levels
themselves are harmful, but researchers have
calculated, for example, that 1200ppm CO
2
means that 2% of the air in the room was inhaled
by someone else already (doi.org/10.1034/j.1600-
0668.2003.00189.x).
At 1000ppm, we should start opening the door
and the window, getting some fresh air in, and
when you do that, the CO
2
level drops back down
fairly quickly. “The wind is blowing into the room
and moving the aerosols and the CO
2
quickly out
of the room,” said Guido.
Then the window and doors are closed,
perhaps because the room temperature becomes
uncomfortable.After about an hour, the CO
2
level
rises again to previous levels.This time, a window
is opened just a little bit.The CO
2
level falls, but
slowly, because the amount of fresh air coming
in through the window is not sufficient.“This
could take an hour to get fresh air conditions in
the room,” said Guido.The CO
2
device can give
feedback on whether the conditions are changing
in the room and how quickly that happens.
A CO
2
device can also be used to measure air
quality in an air-conditioned room.If the AC is
only recirculating the air in the room, then CO
2
levels will rise.If it is exchanging air in the room,
then CO
2
should remain lower.
CO
2
Sensors
There are several options for CO
2
sensors,
differing in price and performance. One of the
interesting things about CO
2
sensors is that they
react within seconds to changed conditions.If you
breathe anywhere near the sensor, it will report
the increase in CO
2
almost immediately. Sensors
also need to be calibrated, which can be as
simple as testing them outdoors first.
Andreas Weise / factum Stuttgart Fotojournalismus
Poorly Ventilated
Rooms Spread
Disease
A 2019 article from researchers in Taiwan
looked at the effect of room ventilation on the
spread of tuberculosis during an outbreak on
a university campus.
Tuberculosis is an airborne disease
which spreads through infectious aerosol
generated by patients during cough. In an
indoor environment, infectious aerosol
progressively accumulates and put
everyone in the room at risk unless the
indoor air is continuously replaced with the
fresh outdoor air by ventilation.
This study provides the first empirical
data showing that improving indoor
ventilation to levels with CO
2
<1000 ppm is
highly effective in controlling a TB outbreak
which occurred in poorly ventilated indoor
environment.
Chun-Ru Du, Effect of ventilation
improvement during a tuberculosis
outbreak in underventilated university
buildings, ncbi.nlm.nih.gov/pmc/
articles/PMC7217216
The researchers found that poorly ventilated
rooms at the university were registering
levels above 3000ppm but when the CO
2
in
the room was lowered below 600ppm, the
tuberculosis outbreak stopped.
27
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